Decay Properties of the Nuclides Fermium-256 and -257 and Mendelevium-255, -256, and -257

The decay properties of fermium and mendelevium nuclides produced by ${\mathrm{B}}^{11}$, ${\mathrm{C}}^{12}$, and ${\mathrm{C}}^{13}$ ion bombardments of californium have been studied. Those observed for ${\mathrm{Fm}}^{256}$, ${\mathrm{Fm}}^{257}$, ${\mathrm{Md}}^{255}$, and ${\mathrm{Md}}^{256}$ agree well with previously reported results. For the first time alpha particles were recorded in the decay of ${\mathrm{Fm}}^{256}$ (6.85 MeV) and ${\mathrm{Md}}^{256}$ (7.17 MeV). In the course of the experiments the nuclide ${\mathrm{Md}}^{257}$ was discovered. It decays with a half-life of about 3 h with the emission of 7.07-MeV alpha particles. Evidence is presented that its main mode of decay is electron capture. Attempts to produce and identify the isotope ${\mathrm{Fm}}^{258}$ failed. A brief discussion of the results is given.